ALL may not be well with one of cosmology鈥檚 most prized theories. Last week two teams studying the 鈥渆cho鈥� of the big bang independently reported results that cannot be explained by the theory of inflation, which says that an ultra-fast period of expansion followed the big bang. This brings to four the number of groups whose results question the theory.
鈥淭his could be the first hint that there is something wrong with inflation,鈥� says David Spergel, spokesman for NASA鈥檚 WMAP space observatory, which measures the microwave background radiation left over from the big bang.
The theory of inflation says that a period of exponentially fast expansion magnified the quantum fluctuations in energy density in the early universe, creating the large clumps of matter we see today. A series of cosmological experiments going back to the COBE satellite in 1992 seemed to confirm the theory.
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But last week, two teams using ground-based radio telescopes to measure the temperature of the sky reported something unexpected. When the temperatures of small regions of the sky are compared, there is less variation than between larger regions. The variation falls again for the very largest regions. This does not fit with inflationary theory, which predicts the same variation in temperature no matter what size regions you look at.
Both anomalies fall short of being considered statistically significant, with the probability of seeing either by chance about 1 in 20. 鈥淲e don鈥檛 want to overemphasise this,鈥� says Dick Bond of the University of Toronto in Ontario, Canada, who worked on data from the Cosmic Background Imager (CBI) at Atacama in Chile ().
A second team taking data from the Very Small Array (VSA) in the Canary Islands saw the same effect (). Although neither result is convincing by itself, Spergel says it is intriguing that more than one analysis is seeing the same thing. 鈥淚t鈥檚 suggesting that something is going on,鈥� he says.
One question mark over the independence of the two studies is that both combine their own data with data from WMAP, which taken on its own also hints that the universe鈥檚 temperature variations might be different on different scales. But the CBI and VSA teams point out that the effect increases when their data is added in. And in a fourth analysis, Uros Seljak鈥檚 team from Princeton University saw something similar when they combined data from WMAP with data from 3000 quasars in the Sloan Digital Sky Survey.
All four teams continue to collect data. If the results are confirmed, cosmologists face a choice between two unappealing alternatives. One is to find a different theory of the universe鈥檚 expansion that explains why it looks different at different scales. 鈥淲e don鈥檛 have a theory like that,鈥� says Bond. The other is to fine-tune inflation by arranging for the field that causes it to fluctuate in just the right way. But without any new ideas to explain why the field should do this, making such a change wouldn鈥檛 be convincing. 鈥淚f you are willing to just put something in, you can fit whatever you want,鈥� says Mathias Zaldarriaga of Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.